The proposed research is intended to elucidate the mode of delivery of Vitamin A (retinol) from the blood to the pigment epithelium and neural retina. My recent research has shown that the choroidal surface of bovine pigment epithelial cells has a specific receptor for plasma retinol-binding protein. This receptor is a cell membrane receptor of high affinity. I will attempt to purify this membrane receptor and characterize its interactions with plasma retinol-binding protein. Recently I found that pigment epithelial and photoreceptor cells also have a soluble cytosol retinol-binding protein. I propose to investigate the possibility that retinol bound to the pigment epithelial cytosol-binding protein is secreted from these cells and is adsorbed onto specific receptors on the cell surface of photoreceptors, and that the reverse transport of either retinol or retinal takes place by a similar mechanism, i.e., the retinol photoreceptor cytosol-binding protein is secreted and taken up by the pigment epithelial cells. The validity of the proposed mechanism will be tested by purifying and characterizing the cytosol retinol-binding proteins, by measuring both in vitro and in tissue culture whether they are secreted from pigment epithelial and photoreceptor cells and by testing whether pigment epithelial cells have a specific cell surface receptor for photoreceptor cytosol retinol-binding protein and whether photoreceptor cells have a specific cell surface receptor for pigment epithelial cytosol retinol-binding protein.